A semiconductor device or a liquid crystal display device is manufactured by the technique known as photolithography, in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in this photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and it transfers the pattern of the mask to the substrate via a projection optical system while sequentially moving the mask stage and the substrate stage. In recent years, higher resolutions for projection optical systems have been in demand to deal with farther high integration of device patterns. The resolution of the projection optical system becomes higher the shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system. For this reason, the exposure wavelengths used in the exposure apparatus are becoming shorter year by year, and the numerical apertures of projection optical systems are also increasing. In addition, the mainstream exposure wavelength at present is the 248 nm of a KrF excimer laser, but a shorter wavelength, the 193 nm of an ArF excimer laser, is also coming into practical application. In addition, when exposure is performed, the depth of focus (DOF) is also important as well as the resolution. The resolution R and the depth of focus δ are restively expressed by the equations below.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Here, λ is the exposure wavelength NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. Based on Equation (1) and Equation (2), it is apparent that when the exposure wavelength λ is made shorter to increase the numerical are NA in order to increase the resolution R, the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to match the substrate surface to the image plane of the projection optical system, and there is concern that the focus margin during the exposure operation will be inadequate. Therefore, the liquid immersion method disclosed in Patent Document 1 below, for example, has been proposed as a method of effectively shortening the exposure wavelength and widening the depth of focus. This liquid immersion method fills the space between the lower surface of the projection optical system and tee substrate surface with a liquid such as water or an organic solvent to form a liquid immersion area, and it uses the fact that the wavelength of the exposure light in liquid becomes 1/n that in air (n is normally approximately 1.2 to 1.6 at the refractive index of the liquid) to increase the resolution as it increases the depth of focus by approximately n times.
Patent Document 1: PCT International Publication No. 99/49504